1. Field of the Invention
This invention relates to use of high molecular weight anionic polymers in the deliquoring of aluminum trihydroxide.
2. Description of the Prior Art
Production of chemical alumina compounds in the gel and hydrate forms is most commonly accomplished through the use of the Bayer process or the Bayer-Sinter process. The principal aluminous ore is bauxite, which is a broad term applied to any ore specie composed primarily of hydrated aluminum oxides. Monohydrates and hydrates are the chief hydrated forms and are generally found in the presence of iron, silicon and titanium oxide impurities. See Raymond E. Kirk and Othmer, D. F., Encyclopedia of Chemical Technology, Volume 1, First Edition, 1947, pages 600-602 for additional details.
In the Bayer process, bauxite ore is digested in steel autoclaves or tubular reactors with hot aqueous caustic soda. The result is the formation of sodium aluminate from the reaction of the caustic soda with the monohydrated and trihydrated forms of the aluminum oxides: ##STR1## See Raymond E. Kirk and Othmer, D. F., Encyclopedia of Chemical Technology, Volume 2, 1978, pages 140-143.
After digestion, the second step in the refining of bauxite is removal of the insoluble metal oxide impurities and sodium aluminum silicates. In the Bayer-Sinter process (used where the ore has a high silica content), the sodium aluminum bound with silicates is further treated with limestone and soda ash to produce a dicalcium silicate and free the sodium aluminate for recovery.
These impurities or "red-muds" are difficult to remove because of their small particle size. In many operations chemical additives such as high molecular polymers and natural starches are added to flocculate the impurities and provide faster settling and cleaner supernatant liquors. The impurities are then washed in a continuous counter current decantation.
After all impurities have been removed through washing and filtration, the filtered liquors are cooled and precipitated. Sodium aluminate is hydrolyzed to yield a slurry of aluminum trihydroxide and caustic liquor: EQU AlO.sub.2.sup.- +2H.sub.2 O.fwdarw.Al(OH).sub.3 +OH.sup.-
To speed the precipitation process, the aluminate liquor is seeded with fine crystals washed from previous precipitations. This seeding facilitates easier washing and separation by effecting larger crystal growth.
Once the aluminum trihydroxide is precipitated, it is processed through a series of mechanical deliquoring and repulping steps where wash water is applied to remove excess sodium ion from Al(OH).sub.3 filter cakes before drying. Rotary drum vacuum filters are most frequently used, although belt filters and centrifuges are possible alternatives. The aluminum trihydroxide cakes produced from the dewatering steps are fed to dryers where calcination of the product takes place.